Language: struct literals

struct Vec2 { x: u8, y: u8 }
    var pos: Vec2 = Vec2 { x: 100, y: 50 }
    on frame {
        pos = Vec2 { x: pos.x + 1, y: pos.y }
    }

- AST: new \`Expr::StructLiteral(name, fields, span)\` variant
- Parser: in expression position, \`Ident {\` enters struct-literal
  mode when the new \`restrict_struct_literals\` flag is off.
  \`if\`/\`while\`/\`for\` conditions set the flag so the \`{\` keeps
  going to the following block. Condition contexts can still use
  struct literals by parenthesizing them.
- Analyzer: validates that the struct type exists, each named field
  belongs to it, and each field value has a compatible type.
- IR lowering: desugars \`var = StructLiteral { ... }\` (both in
  assignments and variable initializers) into per-field StoreVar
  operations against the analyzer-synthesized \`var.field\`
  variables. No IR type for struct values is needed.
- AST codegen: no-op (legacy path).
- examples/structs_enums_for.ne now uses a struct literal for the
  initial \`player\` state instead of per-field assignments.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

examples/structs_enums_for.ne

@@ -28,7 +28,16 @@ struct Player {
     alive: bool,
 }
 
-var player: Player
+// Struct literal initializer in declaration.
+var player: Player = Player {
+    x: 120,
+    y: 112,
+    vx: 0,
+    vy: 0,
+    facing: Down,
+    frame: Idle,
+    alive: true,
+}
 
 // A small fixed-size array of enemy x-positions. In a real game this
 // would be an array of structs once those are supported.
@@ -38,15 +47,6 @@ var enemy_y: u8 = 100
 const SPEED: u8 = 1
 
 on frame {
-    // Initialize the player once on the very first frame.
-    if player.alive == false {
-        player.x = 120
-        player.y = 112
-        player.facing = Down
-        player.frame = Idle
-        player.alive = true
-    }
-
     // Read controls and update position. Velocities are u8 so we
     // treat them as signed by adding/subtracting SPEED.
     if button.left {